This present invention relates generally to manufacturing objects. More particularly, the invention relates to a method and structure for fabricating a spatial light modulator with a high fill factor. Merely by way of example, the invention has been applied to the formation of a spatial light modulator having a torsion spring hinge and mirror plate positioned in the same plane with a uniform reflective layer covering the torsion spring hinge. The method and device can be applied to spatial light modulators as well as other devices, for example, micro-electromechanical sensors, detectors, and displays.
Spatial light modulators (SLMs) have numerous applications in the areas of optical information processing, projection displays, video and graphics monitors, televisions, and electrophotographic printing. Reflective SLMs are devices that modulate incident light in a spatial pattern to reflect an image corresponding to an electrical or optical input. The incident light may be modulated in phase, intensity, polarization, or deflection direction. A reflective SLM is typically comprised of an area or two-dimensional array of addressable picture elements (pixels) capable of reflecting incident light. A key parameter of SLMs, especially in display applications, is the portion of the optically active area to the pixel area (also measured as the fraction of the SLM's surface area that is reflective to the total surface area of the SLM, also called the “fill ratio” or “fill factor”). A high fill factor is desirable for some applications.
Some conventional SLMs utilize designs that include substantial non-reflective areas on their surfaces, reducing the SLMs reflective efficiency. Another problem that reduces reflective efficiency with some SLM designs, particularly in some top hanging mirror designs, is large exposed hinge surface areas. These exposed hinge surface areas result in scattering and diffraction due to the exposed hinge structure, which negatively impacts contrast ratio in display applications, among other parameters.
Other conventional SLMs require multiple layers including a separate layer for the mirrors, hinges, electrodes and/or control circuitry. Manufacturing such a multi-layer SLM requires the use of multi-layer thin film stacking and etching techniques and processes. Use of these techniques and processes may be expensive and produce lower than desired yields. For example, the use of these techniques sometimes involves extensive deposition and removal of sacrificial materials underneath the surface of the mirror plates. Multi-layer thin film deposition and stacking underneath the surface of the mirror plate typically results in rougher mirror surfaces, thereby reducing the reflective efficiency of the mirrors. Moreover, having the mirror and the hinge in a different layer or substrate results in translational displacement upon deflection of the mirror. With translational displacements, the mirrors in an array must be spaced to avoid mechanical interference among adjacent mirrors. Because the mirrors in the array cannot be located too closely to the other mirrors in the array, the SLM suffers from a lower than optimal optically active area or lower fill factor.
Thus, there is a need in the art for a spatial light modulator with improved reflective efficiency, SLM device long-term reliability, and simplified manufacturing processes.